In a colored cathode ray picture tube a shadow mask or aperture mask is located between the electron guns at the rear of the tube and the phosphor mated face plate at the front of the tube. Electron beams pass through tiny openings or apertures in the shadow mask and impinge upon suitable color producing phosphor dots, a triad one dot for each of the three primary colors. During operation of the picture tube the shadow mask openings are used as a guide for the electron beams.
In order to manufacture the shadow masks for use in the cathode my tubes that are used in color monitors it is necessary to proceed through a number of steps to etch a metal web into a shadow mask having the small precision openings of proper dimensions and proper location. In a typical shadow mask manufacturing process one makes a photographic printing plate with a glass base. The photographic printing plate is referred to as a photo printing plate and it contains a master pattern for use in projecting an etchant resist pattern in a layer of etchant resist such as caesin resist which is located on the surface of a metal web. Subsequently, openings are etched in the metal web and the metal web is separated into individual shadow masks which are stacked for batch annealing.
One of the problems with etching shadow masks is how to make the openings in the mask or the acuity as sharp as possible without having the shadow masks adhere to each other during the batch annealing process. Generally, the greater the roughness and skewness on the surface of the mask the less the acuity of the apertures in the mask. On the other hand, unless the roughness on the surface of the shadow mask exceed certain levels the shadow masks adhere to each other during the batch annealing process resulting in destruction of the shadow masks.
One method of overcoming the seizing process during annealing is described in Derwent Abstract of patent application. The abstract describes shadow masks which are annealed and stacked. To prevent the shadow masks from seizing during annealing the inventor imparts a minimum specific roughness and specific skewness to the opposite sides of the metal. To avoid seizing of the masks the Dement Abstract suggests that the average centerline roughness (R.sub.a) should have a value between 0.3 to 0.7 microns. Unfortunately, such surface roughness values produce aperture masks with less overall acuity.
The present invention provides a means for producing shadow masks with high acuity, while still permitting batch annealing of the shadow mask. To obtain the high acuity while maintaining the ability to batch anneal the present invention utilizes shadow masks with the surface roughness on the cone side sufficiently larger than the surface roughness on the grade side of the shadow mask to allow gasses between the stacked masks during the annealing process. The surface roughness on the grade side of the mask is sufficiently small so that when the mask is printed and etched, the apertures have a high acuity. By having a larger portion of the surface roughness on the cone side of the mask then on the grade side of the mask one can maker a high acuity opening in the aperture mask while still maintaining the ability to batch anneal the shadow masks.